Tick-borne bacterial pathogens of humans cause significant morbidity and mortality throughout the United States and abroad. Lyme disease (LD), caused by Borrelia burgdorferi, is the most prevalent arthropod-borne disease of humans in the United States and many other countries throughout Europe and Asia. Tick-borne relapsing fever (TBRF), caused by Borrelia hermsii, is endemic in scattered foci throughout many regions of higher elevation in the western United States. Our work has focused on two areas: 1) to improve on the serodiagnosis of LD and TBRF fever by using recombinant DNA technology to clone genes of spirochetes that express proteins that induce specific and detectable antibody responses; 2) to examine how spirochetes adapt to their tick and mammalian hosts. This work requires that we maintain colonies of Ixodes scapularis and Ornithodoros hermsi, respective tick vectors of LD and TBRF spirochetes, and infect these ticks via a laboratory mouse - tick cycle. Our interests in serodiagnosis and adaptations associated with tick infection and transmission demand that we have a solid understanding of genetic and phenotypic variation within our species of interest. During the last 16 years we have made an effort to acquire new, low passaged isolates of B. hermsii. Our three most recent isolates of B. hermsii were made in the summer, 2002, from the blood of patients exposed to infected ticks in a log houses in northern Idaho and western Montana. We now have 31 isolates of this species, which have come from localities throughout a major part of the known distribution where relapsing fever has been associated with the occurrence of the tick vector, Ornithodoros hermsi. DNA sequence analysis of all the isolates is complete and includes complete sequences of 5 genes: 16S rRNA, flaB, gyrB, glpQ, and vsp33. Concatenated sequences of 16S rRNA, flaB, gyrB, glpQ were assembled for each isolate and analyzed by distance matrix and neighbor-joining methods. This analysis segregated the isolates into two distinct clusters, which we have designated Genomic Group I and Genomic Group II. The two genomic groups were also distinct from other species of relapsing fever spirochetes for which we sequenced the same genes. Genomic Group I contains 19 isolates and Genomic Group II contains 12 isolates. The two genomic groups are sympatric in part of their ranges and isolates from each group originated from similar areas of Washington, Idaho and California. Amino acid values for Vsp33 varied from 58 to 100%. These results demonstrated that all isolates of B. hermsii have Vsp33 and that it is likely under selective immune pressure to vary during the initial infection following transmission by tick bite. We have nearly completed the DNA sequencing of the linear chromosome of Borrelia hermsii and Borrelia turicatae. Analysis has identified 22 genes on their chromosome that are absent in the genome of the Lyme disease spirochete, Borrelia burgdorferi. Pathways for glycerol-3-phosphate metabolism and purine salvage are unique to the relapsing fever spirochetes and may be involved in the diffence in pathogenicity of the relapsing fever spirochetes that allows them to achieve high cell densities in the host's blood.